Multilayered sheets with layers of different plastic materials have been coextruded in the past. The materials for the layers are selected to provide different properties (e.g., oxygen barrier, moisture barrier, thermoformability, heat sealability, heat resistance) depending upon the particular application for the sheet.
In selecting materials for the layers of the sheets, one is often seriously constrained because the different layers must be made of materials that adhere to other layers in the sheets, and some materials should be in interior layers protected from moisture. Also, when the sheet is to be formed into food containers, the food contacting layers must be made of material approved by the Food and Drug Administration. The resulting sheets may thus have rigid materials placed in the center of the sheets owing to such design constraints, and if a more rigid sheet is desired, the layers must be made thicker. For example, acrylonitrile-methyl acrylate copolymer provides excellent oxygen barrier properties and is very rigid; however, it is often coextruded as an interior layer because it has not been approved for food contact by the Food and Drug Administration.